This invention generally pertains to injection molding apparatus. More specifically, the present invention relates to a fluid assisted injection molding apparatus which utilizes a pin positioned in the sleeve such that a fluid flow passage is defined between the pin and the sleeve.
The invention is particularly applicable to a gas assisted injection molding process in which a nozzle is utilized to inject a viscous fluid such as a molten plastic into a mold cavity and a non-viscous fluid such as a gas is also injected to the mold cavity through a separate opening. However, it will be appreciated by those skilled in the art that the invention has broader applications and may also be adapted for use in many other injection molding environments where both a relatively viscous fluid, such as a plastic or wax, and a relatively non-viscous fluid such as a gas or liquid are injected into a mold cavity.
Recently, gas assisted injection molding has gained popularity. In this process, the mold cavity is filled with a plasticized thermoplastic material, usually to a volume less than 100% of the mold cavity, and inert gas is injected under pressure into the plasticized material to fill the rest of the volume in the mold cavity. The gas runs along specially designed channels in the mold so as to create a continuous network of hollowed-out sections in the molded product. Gas assisted injection molding allows for a reduction of stress and warpage, elimination of sink marks, and the provision of smooth surfaces on the injection molded part. In addition, clamp tonnage requirements can be reduced in comparison to conventional injection molding processes. The process also permits the use of different wall thicknesses for a single part and faster cycle times in comparison to conventional injection molding processes. Also, gas assisted injection molding reduces the need for external flow runners.
A variety of nozzles which regulate the flow of both plastic and gas are known. However, for the production of some moldings, especially moldings of complex design, it is desirable to introduce the pressurized gas or other relatively non-viscous fluid at a different location than that at which the plastic is introduced, or perhaps at several locations, all of which are spaced from the plastic injection point. Such a situation may arise, for example, when it is desired to employ a hot runner system where the choice of gate positions is restricted by the desire to avoid an overabundance of gas channels which otherwise would need to be connected to one another.
Certain sprue bushing arrangements are known which enable one to retrofit a gas injection system onto a conventional plastic injection molding machine without extensive modifications. Several in-article gas-injection systems are also known. One major problem, with the known gas assisted injection molding nozzles, the known in-article gas injection pins, as well as with the known gas-injection sprue bushings, is the plugging of the gas channels in these devices with thermoplastic during the venting of the gas. In addition, the gas piping and valves downstream from the apparatus can become plugged. The apparatus then becomes useless until the channels are cleaned out which is a time consuming, difficult, and expensive process.
Accordingly, it has been considered desirable to develop a new and improved injection molding apparatus for in-article and in-runner fluid injection as well as sprue bushing type fluid injection which would overcome the foregoing difficulties and others, while providing better and more advantageous overall results.